1. Field of the Invention
The invention relates to a method and a device for joint-welding coated metal sheets in a butt joint.
2.Description of Related Art
Hot-forming steel components has become well-established in automobile construction in recent years. In this context, many tailored products are manufactured which consist of a plurality of plates of different sheet thicknesses and/or material qualities welded together and which offer high load capacity at relatively low sheet thicknesses and thus a low weight. The plates provided for hot-shaping and press-hardening are generally coated with an organic or inorganic protective layer and/or corrosion protection layer, so as to prevent the workpiece from scaling when hot prior to the hot-forming. In this context, an aluminium alloy is usually used as the coating material, preferably an aluminium-silicon alloy. However, when coated metal sheets of this type are welded, there is often the problem that coating constituents, in particular aluminium, end up in the weld melt and together with iron in the weld seam form Fe-Al compounds, which have a relatively low strength and can weaken the component at the weld seam or even cause the component to fail. Zinc coatings are also possible. In this case, there may be the problem that, during welding, zinc is deposited at the grain boundaries, potentially reducing maximum tensile and compressive stresses in the weld seam.
To solve this problem, what are known as stripping methods have previously been proposed, in which the coating, for example of metal, is removed at the edges of the sheet metal borders to be welded (see for example DE 20 2007 018 823 U1). However, these methods are complex and expensive.
It is further known in the art to add a welding additive to the weld melt so as to improve the strength of the weld seam or of the manufactured component. Thus, for example, JP 07041841 A discloses a laser welding method for joining steel sheets arranged in a butt joint, in which fine carbon powder is added to the weld melt so as to achieve a carbon-enriched weld melt and thus a harder weld seam.